1. Field of the Invention
This invention relates to processes for propagating plants. More particularly, the invention relates to processes for handling, sowing, and germinating plant somatic embryos.
2. Description of the Related Art
Considerable attention has been given to the development of somatic embryogenesis processes for clonal reproduction of plants, and consequently, the specific steps of somatic embryogenesis have been documented in the art for a wide diversity of plant species including both gymnosperms and angiosperms. All methods of somatic embryogenesis are known as tissue culture processes and generally commence with the selection of an explant from a desired plant. The explant is removed from the parent plant tissue by excision and is subsequently cultured on at least one medium to produce a cell mass capable of further differentiation and development. The cell mass can be maintained and proliferated in the undifferentiated state indefinitely, or manipulated to stimulate differentiation into immature somatic embryo structures which can then be cultured further into mature embryos (see, for example, U.S. Pat. Nos. 4,957,866; 5,238,835; 5,294,549; 5,491,090; 5,501,972; 5,563,061; 5,677,185, as well as PCT Publication No. WO 96/37096, all of which are hereby incorporated by reference). Matured somatic embryos can be harvested and germinated immediately, or dried and then germinated, or dried and stored until required for germination (for example, refer to U.S. Pat. Nos. 5,183,835; 5,238,835; 5,413,930; 5,464,769, as well as PCT Publication No. WO 96/37095, all of which are hereby incorporated by reference).
Tissue culture media used to proliferate and propagate plant cultures through the various stages of somatic embryogenesis are typically enriched with mixtures of nutrients that are specifically formulated for each plant species and for the various stages of somatic embryogenesis. A common problem encountered with all somatic embryogenesis processes is microbial, i.e., bacterial, fungal, yeast, contamination of the media and/or plant explants and/or the resulting embryogenic cultures. Microbial contaminants compete with the embryogenic cultures for the nutrients in the media, and in many cases will infect, consume, parasitize, or otherwise pathogenize the cultures. Consequently, steps must be taken to prevent microbial contamination from the beginning of the embryogenesis process when the tissue explants are excised from the parent tissues, through production, harvesting, drying and germination of the somatic embryos and their subsequent growth into fully functional transplants, i.e., somatic seedlings which can be transplanted into soil or horticultural growing mixes. All manipulations of the cultures at each step of the somatic embryogenesis processes are typically done using aseptic techniques. Embryogenic cultures which show any evidence of microbial contamination at any step in somatic embryogenesis process are sterilized and discarded.
Two of the greatest barriers to commercializing somatic embryogenesis technologies are the processes of sowing and germinating plant somatic embryos. Although numerous protocols are known for the sowing and germination of somatic embryos and growing them into intact functional seedlings, none of these protocols have demonstrated compatibility with conventional high-volume through-put horticultural equipment and practices.
Generally, the known protocols for germinating somatic embryos fall into two categories. The first is sowing naked, i.e., uncoated, somatic embryos using aseptic techniques, onto sterilized semi-solid or liquid media contained within a solid-support to facilitate germination (e.g., U.S. Pat. Nos. 5,183,757; 5,294,549; 5,413,930; 5,464,769; 5,506,136) and subsequently, transplanting the germinants into conventional growing systems. The most significant disadvantage of such protocols for sowing naked somatic embryos is that each embryo typically must be handled and manipulated by hand for the germination and transplanting steps. Although various automation options including robotics and machine vision, have been assessed for their usefulness in cost-effective reduction or elimination of the extensive hand-handling currently necessary to sow naked embryos (Roberts et al., 1995), no commercial equipment currently exists which can reliably, aseptically, and cost-effectively perform the in vitro protocols for germination of naked somatic embryos and subsequent transplanting into conventional propagation systems
The second category of protocols teach encapsulation of somatic embryos (e.g., U.S. Pat. Nos. 4,777,762; 4,957,866; 5,183,757; 5,482,857, all of which are herein incorporated by reference) to provide a means by which the embryos can presumably be sown with mechanical devices such as seeders and fluidized drills, into conventional growing systems. However, there are a number of disadvantages with encapsulated somatic embryos. For example, the hydrated semi-solid physical characteristics of encapsulated embryos make them incompatible for use with conventional seeding equipment currently available for commercial plant propagation, because the semi-solid encapsulated somatic embryos tend to clump together during handling and consequently, are difficult to singulate and dispense. Furthermore, compositions of encapsulated embryos prepared as taught by the art, clog-up the conventional equipment, and for these reasons, it currently is not possible to sow encapsulated embryos with conventional seeding equipment. Consequently, novel equipment has been developed specifically for delivery of encapsulated somatic embryos into conventional growing systems. Such sowing devices have been reviewed by Sakamoto et al. (1995), but these devices have only been developed and tested as prototypes. Because of mechanical limitations and the high costs associated with the prototype mechanical seeders developed for sowing encapsulated embryos, none are currently available for commercial acquisition and use.
Another disadvantage with encapsulated somatic embryos is the lack of nutrient availability that is characteristically supplied to zygotic embryos by their attendant endosperm or megagametophyte tissues. Consequently, the encapsulation technology for somatic embryos has been extended to include the incorporation of various nutrients such as sugars, fertilizers, oxygen, into the encapsulation media (e.g., Carlson & Hartle, 1995; U.S. Pat. Nos. 4,583,320; 5,010,685; 5,236,469, all of which are herein incorporated by reference). However, a distinct disadvantage associated with nutrient-amended encapsulated embryos is their susceptibility to microbial invasion during manufacture, storage, and during germination if germinated on non-sterile media.
Furthermore, it must be pointed out that although considerable prior art (e.g., PCT Patent Application WO 94/24847, and U.S. Pat. Nos. 5,010,685; 5,236,469; 5,427,593; 5,427,593; 5,451,241; 5,486,218) teaches methods to manufacture “artificial seeds” consisting of somatic embryos encapsulated in gels, which may or may not be amended with nutrients, and which may or may not be encased within a rigid covering, and although the prior art makes references to sowing said artificial seeds ex vitro into germination media comprised of soil or soil-less mixes, the prior art only teaches methods for germinating said artificial seeds in vitro, i.e., on sterilized semi-solid laboratory media. No methods are taught in the prior art for sowing said encapsulated somatic embryos and/or manufactured and/or artificial seed into conventional growing systems using conventional sowing equipment.
However, the most significant disadvantage with all prior art taught for encapsulating or otherwise coating somatic embryos, is that somatic embryos processed by the use of those protocols typically have as a consequence, much lower germination vigor and success than corresponding zygotic seeds (Carlson & Hartle, 1995). Carlson and Hartle (1995) concluded that considerable research is still required before “manufactured” or “artificial” seeds based on encapsulation and/or coating of somatic embryos will have practical utility. However, it should be noted that the germination vigor of naked, i.e., uncoated or non-encapsulated somatic embryos produced with methods disclosed in the art can approximate those of corresponding zygotic seeds (e.g., greater than 85%) (Gupta & Grob, 1995).